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Prior to taping Monday's episode of "The Ellen DeGeneres Show," Chris Evans – her co-star in "Avengers: Age of Ultron" and "Captain America: Civil War" – hid in the bathroom of her dressing room and scared her silly. Ellen DeGeneres replayed the footage for them later on, but not before Olsen remembered that Evans had done something similar to Scarlett Johansson last year. "Why didn't I think of that?" she asked. Evans told the host, "How can you resist? The best part about your show is you endorse this type of behavior. I really, really am addicted to it."

But first: the Olsen prank! Thanks to DeGeneres' staff, Olsen was lured into the bathroom. "I was told that there were some QVC products in the bathroom to check out. I was like, 'Oh, is that why you did the thing with Julia Roberts the other day?'" the actress said. Little did she know, Evans was crouching down, ready to surprise her the moment she stepped through the door. After seeing her terrified reaction, Olsen laughed and said, "I have a really bad fight or flight response, I think."

"Well, we've been dating like secretly for the last three years," Olsen said, joking, "We're actually engaged." When reports surfaced that they were seeing each other romantically, Evans sent Olsen a text saying, "Hey, guess what? Apparently we're dating." Olsen replied, "Oh, my God! That's really exciting for us." Evans joked that they dated "real on the low" to avoid suspicion. DeGeneres said they "seem compatible," to which Olsen replied, "Maybe we should."

As talk turned to Evans' dog, a staffer dressed as Iron Man burst through DeGeneres' coffee table and scared Evans. "You got me. Could we play it back?" he said after settling down. "I really want to see it."

Check it out in the above video. The two stars also took part in a game of "Last Dance."